As is known, the combustion chambers of internal combustion engines, in particular diesel engines, are provided with heating devices, known typically as “glow plugs”, whose function is appropriately to heat the combustion chambers and the operating fluid in these chambers so as to ensure a certain efficiency of the combustion process, even in operating conditions which are not optimal, for instance at a low temperature of the combustion chamber and/or of the operating fluid.
In internal combustion engines, one of the most critical of the operating conditions to which the combustion process is subject, and in which the use of glow plugs is required, is in particular the engine ignition phase.
In this phase, in practice, the temperature of the combustion chamber is low, i.e. it is lower than the working temperature required to obtain a sufficiently efficient combustion process; a supply voltage is therefore supplied to the glow plug so to bring the temperature of the latter to a value equal to an objective temperature to be reached in working conditions.
It is also known that one of the most important requirements for the drivers of vehicles with internal combustion engines is the need to reduce to a minimum the preheating time of the glow plug, which corresponds to the time interval taken by the plug, during the ignition phase of the engine, to bring its temperature to a value equal to the objective temperature.
For this purpose, open loop electronic control systems adapted to drive the glow plug so as to reduce the preheating time have been proposed in the latest generation internal combustion engines. During the engine ignition phase, these electronic control systems in particular boost the supply voltage of the glow plug, i.e. they increase the supply voltage to a value greater than the nominal voltage supplied to the glow plug in normal working conditions, in order to cause the temperature of the glow plug to increase extremely rapidly, thereby obtaining a reduction of the preheating time.
At the end of the ignition phase, the control system stabilises the supply voltage of the glow plug to the nominal value in order to maintain its temperature at a value substantially equal to the objective temperature.
Although they reduce the preheating time of the glow plug, the electronic control systems discussed above have a number of drawbacks: first, the supply of an overvoltage to the glow plug may damage it when the initial conditions of the plug and the combustion chamber differ from the conditions set in the control; in practice, if the ignition, rather than taking place from a “cold” engine, takes place from a “hot” engine, i.e. at a temperature slightly lower than the working temperature, then the supply of an overvoltage to the glow plug may generate an extremely high temperature which is higher than the temperature that can be tolerated by the glow plug, thereby subjecting the latter to excessive thermal stresses which it is unable to withstand.
A second drawback lies in the fact that the open loop electronic control systems discussed above do not ensure that the plug temperature remains stable enough with variations of those of the engine operating parameters which to some extent cause a change of temperature in the combustion chamber. In other words, the control of the glow plug temperature carried out by the above-mentioned electronic control systems is not very reliable as the temperature parameter to be controlled is conditioned by a number of engine parameters and by a number of environmental conditions to which the plug is exposed.
In order to reduce the preheating time, self-regulating glow plugs have also been proposed and are provided with an internal varistor which varies their resistance as a function of temperature so as to cause an automatic regulation of the thermal power generated, thereby obtaining an automatic control of the temperature of the glow plug.
These self-regulating glow plugs have the drawback that they are subject to a degree of temperature dispersion in the various operating conditions of the engine; in practice, as the engine operating point varies there is a change in the heat exchange and the self-regulating glow plug is unable appropriately to adapt its heating power, and is thus subject to higher temperature variations.
EP-1408233 discloses a process for controlling the heating of glow plugs in a diesel engine comprises emulating the thermal behavior of the plug on heating, and WO-9506203 relates to a method of driving a heating element such as a glow plug from an electrical power supply.